Such selecting methods or procedures are, for example, used in stock maintenance systems. These procedures are referred to as anti-collision methods. Anti-collision methods are used, for example, in contactless identification systems or so-called radio frequency identification (RFID) systems. Such a system normally comprises a base station or a reading device and a plurality of transponders or remote sensors. These transponders or sensors are present simultaneously in the interrogation range or field of the base station. In case the data transmission is to take place between the base station and only one transponder or a particular group of transponders among all of the transponders, then a selection method must be performed prior to a respective data transmission to avoid collisions of simultaneous communications with several unselected transponders.
U.S. Pat. No. 5,856,788 (Walter et al.) describes a method and apparatus for radio frequency identification tags in which the transponders are selected by means of a bit-by-bit comparing of their identification bit sequence with a selection bit sequence. The identification bit sequence is unique or unambiguous and statically allocated. The selection takes place in a half-duplex method in which the base station interrogates the transponders by transmitting a selection bit and the transponders respond to the interrogation by transmitting a respective bit of their identification bit sequence if the respective bit corresponds to the requested selection bit value. The transponders compare the corresponding bit of their identification code bit sequence with the respective requested selection bit. The comparing takes place on the basis of a comparison criterion. The comparison criterium in the case of Walter et. al. is the “equal operator”. More specifically, transponders remain activated if the associated or respective bit of their identification code bit sequence corresponds to the selection bit. Due to the half-duplex method a relatively long time is necessary until one transponder is selected.
The publication “ISO WD 18000-6 Mode 3” of Feb. 1, 2002, entitled “Automatic Identification-Radio Frequency Identification For Item Management” describes a selection method in which the selection also takes place on the basis of an unambiguous identification bit sequence. This conventional selection method operates as a full duplex method, whereby the time duration for the selection is reduced. The transponders are also selected by means of their unambiguous static allocation of the identification bit sequence.
The above described two conventional methods require that an unambiguous identification code bit sequence is allocated to each transponder within the interrogation range or field of the base station. The identification bit sequence is conventionally formed by a so-called unique identification (U-ID). Such U-ID is allocated to the transponders, for example when the transponders are manufactured. The definiteness or unambiguity of the U-ID can, however, not be always guaranteed due to the multitude of U-ID specifications in open systems.